1. Field of the Invention
This invention relates to a composition for imparting curability to a specific photocurable compound as a starting material on exposure to visible light, a method for curing by exposure to visible light a compound prepared as a starting material by incorporation therein of the composition, and a method for the production of a glass fiber-reinforced plastic. The use of the composition of this invention allows the photocurable compound to cure quickly. A cast plate and a glass fiber-reinforced plastic (hereinafter referred to briefly as "FRP") plate formed in a thickness in the range of 1 to 10 mm and cured acquire hardness virtually indiscriminately on the obverse and reverse sides thereof and enjoy perfect curing.
2. Prior Art Statement
It has been known that a radically polymerizable monomer can be cured in the form of incorporating therein a photocuring agent on exposure to ultraviolet rays (hereinafter referred to briefly as "UV") or visible light. Particularly, the practice of using UV for the curing of such films as are used for surface coating and in inks has found wide recognition.
In curing a thick film or FRP having a thickness of more than 1 mm, however, the UV curing is not suitable because the rays are absorbed by the resin or glass fibers and consequently the shaped article is not allowed to be cured to the core.
The practice of utilizing visible light for photocuring the thick film or FRP more than 1 mm in thickness has been known to the art.
The curing method resorting to exposure to visible light has been reported variously as follows.
Japanese Patent Publication SHO 60(1985)-15656, for example, discloses a photocuring agent which is a combination of a derivative of benzoyl oxime carbonate ester, a photoreducing dye such as eosin, for example, and a tertiary amine.
The curing methods using a combination of an .alpha.-diketone and a tertiary amine are disclosed in Japanese Patent Publication SHO 54(1979)-10986, Japanese Patent Publication SHO 57(1982)-59246 and Japanese Patent Publication SHO 60(1985)-8241, for example.
An invention using an acylphosphine oxide as a photocuring agent is disclosed in Japanese Patent Publication SHO 60(1985)-8047. Further, methods using a peroxide in combination with a curing agent have been known to the art. For example, Japanese Patent Publication SHO 57(1982)-202304 discloses a photocuring agent comprising a curing agent combined with benzophenone or benzil, and one of peroxides consisting of t-butyl peroxybenzoate, methyl ethyl ketone peroxide and cyclohexanone peroxide.
The conventional methods mentioned above have their own problems as follows.
When the curing agent combining a derivative of benzoyl oxime carbonate ester, a photoreducing dye and a tertiary amine is used, the curing time required for the resin is as long as or exceeds 20 minutes, the cured resin is deficient in hardness, and the insufficiency of hardness is conspicuous particularly on the side of the cured resin opposite to the side exposed to the light.
In the case of the use of a combination of an .alpha.-diketone and a tertiary amine, the use of an acylphosphine oxide, and the combined use of a photocuring agent and a peroxide, their effective application to FRP is difficult because the curing compounds as a starting material polymerize only in low conversion.
None of the resins using the known curing agents capable of providing desired curing by virtue of exposure to visible light has demonstrated the characteristics in that the curing proceeds quickly and produces a cured film having a large thickness in the range of 1 to 10 mm and that the shaped article of resin produced by the curing exhibits highly desirable properties, particularly high hardness, uniformly throughout the volume thereof and manifests this high hardness virtually indiscriminately on the obverse and reverse sides of the shape article.
Further, in the production of FRP from a polyester resin or an epoxy acrylate resin as a raw material, there is employed a method for curing the resin by mixing the resin with a curing agent and exposing the resultant mixture to a light. None of the known curing agents is practically suitable for this method, however, because the cured shaped article of resin produced by using the curing agent exhibits hardness discriminately on the obverse and reverse sides of the shaped article.
An object of this invention is to provide a composition capable of imparting curability to a photocurable compound as a starting material on exposure to visible light and allowing the photocurable compound incorporating the composition to cure quickly and produce a cured shaped article which, in the form of a film about 1 to 10 mm in thickness, exhibits hardness virtually indiscriminately on the obverse and reverse sides of the shaped article. Another object of this invention is to provide a method for curing the photocurable compound as a raw material by the use of the composition.
Still another object of this invention is to provide a method for the production of FRP which cures FRP more quickly than the methods generally used, the FRP in the cured form exhibiting hardness virtually indiscriminately on the obverse and reverse sides thereof.